In a number of applications, particularly in the automotive industry, it is important to provide high-strength structural members at the lowest possible mass. A number of composite materials have been proposed by others in the past for use in forming structural members, including exotic lightweight alloys, In the automotive industry, however, the need for mass reduction without sacrificing strength must be balanced against the cost of the product to the consumer. Thus, there is a need for maintaining or increasing the strength of structural members such as rockers, windshield, pillars, radiator support beams, drive shafts, side impact beams, and bumpers without significantly increasing materials and labor costs.
The reinforcement of motor vehicle structural members through the use of composite materials is known. For example, the present inventor has disclosed a number of metal/plastic composite structures for use in reinforcing motor vehicles components. In U.S. Pat. No. 4,901,500, entitled “Lightweight Composite Beam,” a reinforcing beam for a vehicle door is disclosed which comprises an open channel-shaped metal member having a longitudinal cavity which is filled with a thermoset or thermoplastic resin-based material. In U.S. Pat. No. 4,908,930 entitled, “Method of Making a Torsion Bar,” a hollow torsion bar reinforced by a mixture of resin with filler is described. The tube is cut to length and charged with a resin-based material.
In U.S. Pat. No. 4,751,249, entitled, “Reinforcement Insert for a Structural Member with Method of Making and Using the Same”, a precast reinforcement insert for structural members is provided which is formed of a plurality of pellets containing a thermoset resin with a blowing agent. The precast is expanded and cured in place in the structural member. In U.S. Pat. No. 4,978,562, entitled, “Composite Tubular Door Beam Reinforced with a Syntactic Foam Core Localized at the Mid Span of the Tube”, a composite door beam is described which has a resin-based core that occupies not more than one-third of the bore of a metal tube.
In addition to the present inventor's own prior work, a number of metal laminates constructions are known in which flat metal plates are bonded together by an intervening layer of resin. It is also known to form a metal laminate sheet for use as a floor panel member which comprises a pair of flat metal sheets having an intervening layer of asphalt or elastic polymer.
Although filling sections with plastic foam does significantly increase section stiffness (at least when high-density foams are utilized), they also increase mass and thus part weight, which, as stated, is an undesirable feature in automotive applications. Moreover, although increasing the metal gauge of a section or adding localized metal reinforcements will increase stiffness, as the metal thickness increases, it is more difficult to form the part due to limitations in metal forming machines. Importantly, in many applications increasing metal gauge will not work effectively because stiffness frequency is proportional to section stiffness divided by section mass:
f≈√{square root over (k/m)} (i.e., frequency is proportional to the square root of stiffness over mass). Mass and stiffness are increased proportionately, with no resultant change in the dynamic stiffness frequency of the part.
In addition, filling a section entirely with foam creates a door beam 100 is heated to a temperature sufficient to thermally expand or “foam” layer 106, such as when the motor vehicle is placed in a paint oven or the like. At the blowing agent activation temperature, the resin that forms layer 106 expands to its final form and solidifies to form a strong bond between inner shell 114 and outer shell 102. Since layer 106 is a structural foam and inner shell 114 is a thin high-strength steel, structural shell 102 is reinforced with a minimum weight increase.
In addition, filling a section entirely with foam creates a large heat sink and requires elaborate sealing operations to close access holes in the stampings. Also, the presence of the foam may interfere with the placement of interior trim panels, wiring harnesses, and hinges.
Accordingly, it would be desirable to provide a low-cost technique for increasing the stiffness of a section without proportionately increasing the mass. The present invention provides sections which have increased stiffness values with no significant increase in mass and without the use of high volumes of expensive resins. In many applications, the present invention reduces vibrations which cause unwanted “shake” of a component.